C.dbd.C in lipids.
The annual production of vegetable oils is about 90.million toils (Mielke 1992), of which about 20% are hardened (hydrogenated). Furthermore, about 2 million tons of marine oils are hydrogenated yearly. The production is spread over the whole industrialized world. Through the hydrogenation, hydrogen is added to the double bonds of the unsaturated fatty acids. The largest part of the oils is only partly hydrogenated. The desired conditions of melting and the desired consistency of the fats are thereby obtained, which are of importance for the production of margarine and shortening. The tendency to oxidation is reduced by the hydrogenation, and the stability of the fats is increased at the same time (Swern 1982).
In the future, the lipids may be modified by methods belonging to biotechnology, especially gene technology, but hydrogenation will certainly remain.
A problem with the hydrogenation processes of today is, that new fatty acids are produced which to a great extent do not exist in the nature. They are often called trans fatty acids, but the double bonds change position as well as form (cis-trans) during the hydrogenation (Allen 1956, Allen 1986).
Natural fats and oils contain cis double bonds almost exclusively. As a cis double bond is activated at the catalyst surface, it may:
(1) saturate, provided two activated hydrogen atoms are available at a distance sufficiently small, or PA1 (2) deactivate and reform the double bond. However, in reforming trans and cis are created at a ratio of about 3.1. PA1 (1) the span of hydrogenation possible, i e the initial iodine value (IV), and PA1 (2) the rate of formation of trans fatty acids in the reactor.
Thus the formation of trans fatty acids is the result of activated hydrogen not being sufficiently available.
In the beginning of a hydrogenation trans fatty acids are only formed, as it proceeds trans fatty acids are being saturated in parallel to the cis double bonds. The saturation of the latter is preferred and in the equilibrium the ratio of trans to cis is about 2:1.
It follows that the amount of trans fatty acids generate a maximum; at the beginning and at the end of the hydrogenation process it is zero. The size of the maximum depends on and increases with two parameters.
As a rule, trans fatty acids are desired from a technical and functional point of view (Swern 1982), but regarding health, their role is becoming more and more questionable (Wahle & James 1993).
A typical state of the art reactor for hydrogenation is a large tank (5 to 20 m.sup.3) filled with oil and hydrogen gas plus a catalyst in the form of fine particles (nickel in powdery form). The reaction is carried out at a low pressure, just above atmospheric (0,5 to 5 bar), and high temperatures (130 to 210.degree. C.). The hydrogen gas is thoroughly mixed into the oil, as this step restricts the reaction rate (Grau et al., 1988).
If the pressure of hydrogen gas is increased from 3 to 50 bar when soya oil is partially hydrogenated (iodine number at the start=135, at the end=70), the content of trans is reduced from 40 to 15%. The position isomerization is also reduced to a corresponding level (Hsu et al., 1989). These results are of no commercial interest, as these conditions enforce a replacement of the low pressure autoclaves by high pressure autoclaves.
According to the "half hydrogenation" theory, the concentration of activated H-atoms on the catalyst surface determines the number of double bonds being hydrogenated and deactivated without being hydrogenated respectively. A lack of activated H-atoms causes a trans- and position- isomerization (Allen 1956, Allen 1986). A lack of activated H-atoms can be the consequence of low solubility of H.sub.2 in the oil, or of a bad catalyst (poisoned or inadequately produced). Thus, the "half hydrogenation" theory corresponds very well to the empirical results (Allen 1956; Allen 1986; Hsu et al., 1989).
It is possible to deodorize and hydrogenate an oil in the presence of CO.sub.2 and hydrogen (Zosel 1976). Zosel describes in detail how to use CO.sub.2 in order to deodorize the oil. However, it must be emphasized that Zosel does not give any hint, that CO.sub.2 should have an influence on the hydrogenation process. Furthermore, Zosel does not touch on the cis/trans problem.
In the experiments of Zosel, the catalyst is surrounded by a liquid phase during the entire process. Zosel does not disclose the composition, but in the light of the other data, we estimate that the liquid phase consists of oil (about 95%), CO.sub.2 (about 5%) and hydrogen (about 0.03%). This phase is far away from a supercritical condition. As a consequence, the velocity of reaction is limited by the concentration of hydrogen on the catalyst surface. The same applies to all traditional hydrogenation reactions where the catalyst is in the liquid phase as well. The velocity of hydrogenation in the experiments of Zosel is about 100 kg/m.sup.3 h, i.e. somewhat lower than in traditional hydrogenation reactors.